There’s a time for everything – a time to relax in your hot tub, a time to invite friends for a hot tub party, a time to add FROG Maintain®, your monthly non-chlorine shock, and eventually, a time to purge.

While purging your hot tub doesn’t top the “Fun things to do this weekend list,” it does top the “Glad I did it and I love my hot tub even more” list.

Purging may seem daunting, but it can be easy, especially if you know:

  • Why you need to purge

  • When to purge

  • How to purge

Woman sits in an outdoor hot tub with her arm hung on the edge of the spa. The sun is setting into the hills in the distance

Why purge your hot tub?

Even if your water and sanitizer levels are in the ideal range, your hot tub can develop biofilm over time inside the pipes.

Biofilm is made up of living, reproducing microorganisms, like bacteria. They form communities called biofilms. They make a sticky substance that holds them together. Different bacteria use different methods to form these communities, which depend on the environment and their specific characteristics.

Biofilm can form in the hot tub’s pipes and plumbing system, so you can’t see it.

Many hot tubs have more than 100 feet of pipe – that’s potentially a lot of biofilm.

If left untouched, biofilm builds up and becomes more and more resistant to any sanitizer. 

Blood, sweat and tears

How does biofilm build up? A combination of reasons. For example, did you know that the average person brings 100 million bacteria into the hot tub water? These bacteria join other living organisms from biofilm to parasites, virus, algae, mold, mildew, and spores.

The average person also contributes waste such as blood, sweat, tears, urine, saliva, mucous and other bodily fluids. The average hot tub user sweats up to three pints per hour at 102 F. But that’s not all.

Personal care products – lotion, soap, deodorants, makeup, hair products, and perfumes – all dissolve off skin and hair and into the hot tub water.

5 bottles of various personal care products in different colors. There is also a pink loofah in the photo.

The Environmental Working Group reports that the average adult in 2023 uses 12 personal care products each day with 112 unique ingredients. Most of these personal care products are shed into the hot tub water upon immersion.

It’s almost impossible to test for and identify biofilm, since it’s nearly microscopic in its early stages.


  • Consumes sanitizer, affects pH and water balance

  • Can harbor harmful bacteria colonies

  • Causes foam and other water problems

  • Reduces pipe diameter in acute cases


Biofilm is a problem because it consumes chlorine and can harbor harmful bacteria. Once biofilm forms, it’s difficult to remove even with increased sanitizer or shock treatments.

Close up image of very foamy hot tub water.

Now for the good news

Biofilm is removable (in most cases) and preventable. With proper purging, your hot tub and water will be crystal clean and clear for months.

What hot tubs are susceptible to biofilm?

Short answer: all hot tubs, including those that are well-maintained. However, some hot tubs can be more susceptible sooner:

  • New hot tubs

  • Hot tubs used a lot – many times a week, and/or are used by many people – if sanitizers and filters are not maintained well

  • Hot tubs that are not maintained well – for example, with old, used filter cartridges, or incorrect or inconsistent use of sanitizers, and not shocked routinely

  • Unused hot tubs – full of water, or even drained (water can be in the pipes)

How do you know when it’s time to purge?

A best practice is to purge your hot tub when you first purchase it.

Hot tubs are “wet tested” at the factory.  Wet Testing is when a hot tub is filled with water and run to make sure there are no leaks.

The hot tubs are drained, but not blown out, meaning that water is in the plumbing for weeks, maybe months before the hot tub is sold and placed in a consumer’s backyard.  This allows for plenty of time to grow biofilm in the pipes.

It’s ideal to purge your hot tub each time before you drain and refill it.

Purging typically works best when the water is hot – that’s the best time to get all the biofilm and any other buildup out of the pipes and plumbing.  Purging every time you drain and refill your hot tub will allow your hot tub to be its most efficient and ensure that your sanitizer works to kill fresh bacteria introduced into the water – not existing or growing bacteria inside the plumbing.

If you don’t purge each time you drain and refill your hot tub, some signs that biofilm buildup is becoming too much for sanitizers and regular shock include:

  • Inability to get rid of foamy or cloudy water no matter what you try

  • Inability to balance the water’s pH

  • Use of more and more chemicals than usual

  • Bad smelling or off-color water

  • A noticeable buildup of scale around your hot tub

  • Film or slime on the water surface or other surfaces

Underwater shot of a cloudy hot tub.

When it’s time to purge

Follow three main steps – shock, purge and drain. You and your hot tub will be happy in no time:

To purge, start with shock

Shocking is part of the purge process. Use a purge product that’s designed to flush biofilm and contaminants out of the pipes. Before draining, add a shock to kill bacteria.

You may want to flush out the pipes with a hot tub flush product formulated to clear all bacteria, biofilm buildup and contaminants.

This is a good time to ask your retailer what products they recommend. Follow manufacturer’s directions on the product(s) you use.

Warning: You’ll likely notice some nasty looking foam, ugly looking water and gunk. That’s good news – you’re getting rid of it! You can remove a lot of it with a small surface skimmer.


Use a purging additive to remove accumulations from the surface and the hot tub’s plumbing lines. Allow it to circulate through the spa overnight to ensure the biofilm is stripped away from the lines. This is essential.

Failing to purge the plumbing lines will allow the biofilm buildup to circulate back through the hot tub water over and over again.


Once you’ve finished the purge process, shut off your hot tub. Drain the water from your hot tub, following the directions in your hot tub owner’s manual. You can always ask your retailer for help.

While the water is draining, clean the hot tub filter – follow directions on the product you use to clean it; most call for a 24-hour soak.

Or replace the hot tub filter cartridge to  ensure that bacteria is not hiding deep in the pleats of the spa filter.

Expert Tips:

Use a submersible pump (sump pump), placed in the deepest part of the hot tub. Plug it in and turn it on. Turn it off when the water is drained – if it runs dry the motor can burn.

Use a wet/dry vacuum or spa wand to drain any remaining water.

When the water has been drained, clean every surface of the hot tub with cleaner your retailer recommends.

Clean hot tub pillows, rails, the water line, inside and around the jets, rails, steps, the sides, and surface of the hot tub to remove any persistent biofilm. Leave no surface behind!

A gloved hand with a sponge wiping the surfaces of a hot tub.

Give the hot tub a final, thorough rinse with a hose, then drain it again. A final rinse will get rid of any residual cleaner, which can cause foam when you refill the hot tub – then you’ll have to drain again. And that’s a draining experience! 

Refill the hot tub with fresh water. Test it for metals, adjust if needed. Follow the start-up routine your sanitizer product or hot tub retailer recommends to balance your water, and then add your sanitizer.

Image of the FROG @ease Floating System ball - both the blue mineral cartridge and the silver SmartChlor cartridge.

Expert advice

If you use FROG® @ease®, you can maintain clean, clear water longer than the usual 3 months that is most often recommended for draining and refilling your hot tub.

That’s because FROG @ease uses up to 75% less chlorine*. FROG @ease contains no cyanuric acid, so there’s no buildup. And it kills bacteria 2 ways – with SmartChlor®, a unique, patented chlorine and FROG Minerals.

Replace the FROG Minerals every 4 months, they work in conjunction with SmartChlor to keep clear, clean water.

Interested in FROG @ease? See this video for easy hot tub start-up directions:

Sources: A Brief Introduction to Biofilms Survey finds use of personal care products up since 2004 – what that means for your health

*Compared to the minimum ANSI recommended chlorine level of 2.0 ppm for a hot tub


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